Engines may be configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. Such engines are known as variable displacement engines (VDE). In some examples, a portion of an engine's cylinders may be disabled during selected conditions, where the selected conditions can be defined by parameters such as a speed/load window, as well as various other operating conditions including vehicle speed. A VDE control system may disable selected cylinders through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves, or through the control of a plurality of selectively deactivatable fuel injectors that affect cylinder fueling. When transitioning between a VDE mode (where one or more cylinders are deactivated) and a non-VDE mode (where all the cylinders are active), the control system may adjust one or more engine operating parameters to reduce disturbances (e.g., torque disturbances) and attenuate the disturbance during the transition.
One example approach for engine control during a VDE transition is shown by Pallett et al in U.S. Pat. No. 7,225,782. Therein, the VDE engine is coupled in a hybrid electric vehicle having an electric motor. When enabling or disabling a cylinder, torque from the motor is varied to compensate for transient changes in engine output torque caused by the enabling or disabling of the cylinder. In particular, the electric motor is controlled to mask any drivability issues associated with VDE transitions and/or poor combustion stability.
However the inventors herein have identified potential issues with such an approach. As one example, combustion stability may be degraded during the transition. Specifically, when transitioning from the VDE mode (or partial cylinder mode) to the non-VDE mode (or full cylinder mode), cylinder load decreases based on the decrease in aircharge. The lighter cylinder loads generally have less stable combustion and the interaction with the transient fuel compensation, and other cylinder conditions that are different than the operating cylinders due to cooling during deactivation, may contribute to less stable combustion during reactivation. If the engine is equipped for exhaust gas recirculation, EGR control used during the transition may exacerbate the combustion issues. In particular, the EGR may continue to interfere with the lighter cylinder load until the EGR delivered to the cylinders has been sufficiently bled down to reduce combustion issues. In some embodiments, charge motion control valves (CMCVs) may be used to adjust the in-cylinder motion of the air-fuel mixture delivered to the cylinder during the transition. High in-cylinder motion results in better mixing, and more stable combustion. However, due to the slow response time of the CMCV (e.g., the CMCV not shutting quickly enough when transitioning to the lower cylinder load), combustion stability may be compromised. The poor combustion conditions can also lead to slow burns or even misfires. Overall, combustion stability and engine performance may be degraded.
In one example, some of the above issues may be at least partly addressed by a method for an engine comprising: selectively deactivating one or more engine cylinders responsive to operating conditions, and during reactivation of the cylinders, operating the reactivated cylinders with split fuel injection. Specifically, the fuel injection of the reactivated cylinders may be transiently shifted to each of an intake stroke and a compression stroke injection for a number of combustion events. In this way, restart combustion stability is improved and torque disturbances during a transition out of a VDE mode of operation are reduced.
In one example, a variable displacement engine may be configured with selectively deactivatable fuel injectors. In response to selected deactivation conditions, such as reduced engine load or torque demand, one or more cylinders may be deactivated and the engine may be operated in a VDE mode. For example, the engine may be operated with half the cylinders deactivated and with the remaining active cylinders operating at a higher cylinder load. During the deactivation, the active cylinders may be operated with fuel delivered as a single intake stroke injection. In addition, due to the higher average cylinder load, the cylinders may be operable with EGR without incurring combustion stability issues otherwise incurred at low engine loads. The use of EGR during VDE operation provides additional fuel economy benefits.
In response to reactivation conditions, such as increased engine load or torque demand, the deactivated cylinders may be reactivated and the engine may resume a non-VDE mode of operation wherein all the cylinders are operated at a lower average cylinder load. In addition, EGR may be stopped (e.g., by closing an EGR valve) due to the engine's reduced EGR tolerance during the reactivation when the cylinders are transitioning to a lower cylinder load. Even though the EGR valve is closed, due to transport delays along the EGR passage, EGR may purge from the air intake system slower than desired, resulting in increased EGR dilution of intake air during the reactivation. Also, during engine operation in the VDE mode, the exhaust catalyst may becomes saturated with oxygen and may need to be regenerated during cylinder reactivation. Therefore, to reduce combustion stability issues arising from the increased EGR dilution of intake air, as well as to expedite catalyst regeneration, during the reactivation, for a number of combustion events since the reactivation, the reactivated cylinders may be operated with fuel delivered as a split fuel injection. For example, fuel may be delivered as at least a first intake stroke injection and a second compression stroke injection. In addition, spark timing may be retarded. A split ratio of fuel delivered in the first intake stroke injection relative to fuel delivered in the second compression stroke injection may be adjusted based on one or more of a duration of the deactivation, a temperature or oxygen loading of an exhaust catalyst coupled downstream of the reactivated cylinders, an amount of spark retard applied, and EGR level at reactivation. By temporarily shifting to a split fuel injection while retarding spark, reactivation of the exhaust catalyst can be expedited, improving exhaust emissions. In addition, combustion stability issues arising from the decrease in individual cylinder load during the transition out of the VDE mode can be better addressed, particularly in the presence of the EGR. The split ratio may also be adjusted based on other engine operating parameters, such as the alcohol content of the injected fuel, to compensate for drivability and engine stumble issues. As such, the use of a split injection and spark retard may be continued for a number of combustion events until the engine speed is at or above a threshold speed where combustion stability is improved (e.g., at or above idling speed) and/or until the EGR is sufficiently bled down.
In this way, by operating reactivated cylinders with split fuel injection for a number of combustion events during a reactivation from VDE mode of engine operation, restart combustion stability of the cylinders is improved. By injecting at least a portion of the fuel during an intake stroke and a remaining portion in a compression stroke, exhaust catalyst regeneration following the VDE mode of operation can be expedited, providing emissions benefits. By further adjusting the split ratio based on an alcohol content of the injected fuel, poor combustion events that may result in a stumble may be reduced. As such, this reduces drivability deterioration from mixed fuel usage. In addition, the use of a split injection during reactivation improves the EGR usage in the active cylinders during the preceding deactivation. Overall, engine performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.